1. Field of the Invention
The present invention relates to a method of preparing a silver halide photographic emulsion and, more particularly, to a method of preparing a silver halide photographic emulsion by performing spectral sensitization by adding a cyanine dye during or before chemical sensitization. The present invention also relates to a method of preparing a negative tabular silver halide photographic emulsion in which 50% or more of a total projected area are accounted for by negative tabular grains with an aspect ratio of 3 or more.
2. Description of the Related Art
Generally, the preparation of silver halide photographic emulsions consists of grain formation, desalting, and chemical sensitization. Spectral sensitization using sensitizing dyes is commonly performed by adding the sensitizing dyes when an emulsion subjected to chemical sensitization is to be dissolved during coating. Recently, for the purpose of improving the sensitivity/graininess ratio of a silver halide photographic emulsion, spectral sensitization has begun to be performed by adding sensitizing dyes during or before chemical sensitization, as disclosed in U.S. Pat. No. 4,433,048.
According to the supplementary tests conducted by the present inventors, it is found that the improvement in the sensitivity/graininess ratio obtained by performing spectral sensitization by adding sensitizing dyes during or before chemical sensitization is remarkable when the amount of the sensitizing dyes added is 50% or more, preferably 60% or more of the saturation coverage of silver halide photographic emulsion grains. That is, when spectral sensitization is to be performed by adding sensitizing dyes during or before chemical sensitization, it is necessary to use the sensitizing dyes in an amount larger than those used conventionally.
It is, however, found that, when a large amount of sensitizing dyes is used, the sensitivity/graininess ratio is improved, but the photographic properties are largely degraded while a silver halide photographic emulsion is aged in the form of a solution. The present inventors have found that this problem is posed because agglomeration of emulsion grains occurs due to the addition of a large amount of sensitizing dyes. It is assumed that this agglomeration of silver halide emulsion grains takes place since sensitizing dyes adsorbing at a high covering rate make gelatin lose its protective colloid nature with respect to silver halide emulsion grains.
The agglomeration of silver halide emulsion grains is conspicuous especially in tabular silver halide photographic emulsions in which 50% or more of a total projected area are occupied by tabular grains with an aspect ratio of 3 or more. This reason is assumed that large, smooth surfaces of tabular grains are readily brought into contact with each other to aggregate.
The problem to be solved by the present invention and, naturally, the method of the invention are conventionally unknown. U.S. Pat. No. 4,656,122 discloses that high-sensitivity, high-contrast reversal images can be obtained by mixing tabular silver halide grains and a fine-grain silver halide with solubility higher than that of the tabular silver halide grains. This disclosure is different from the present invention in respect of the relationship of the silver iodide content between the tabular silver halide grains and the fine-grain silver halide. In addition, the disclosure is a special effect obtained in only a system in which reversal images are formed.
JP-A-3-238444 ("JP-A" means Unexamined Published Japanese Patent Application) discloses that the adsorption of sensitizing dyes is enhanced to improve the aging stability at high temperatures and high humidities by adding to a silver iodobromide emulsion silver halide fine grains with a silver iodide content larger than that on the surface of the emulsion. This invention disclosed in JP-A-3-238444 is based on the dissolution of the silver halide fine grains and is therefore different from the present invention which is based on the addition of the insoluble silver iodobromide fine grains.